Grass carp (Ctenopharyngodon idella) are a type of freshwater fish recognized for their ability to consume aquatic vegetation. These fish originate from the Amur River basin in eastern Asia, spanning regions of Russia and China. Over time, grass carp have been introduced to numerous countries worldwide, primarily as a biological tool for managing unwanted aquatic plants in various water bodies.
Grass Carp Dietary Preferences
Grass carp are herbivorous, feeding primarily on a variety of aquatic plants. However, they exhibit selective feeding habits, showing a distinct preference for softer, pliable, submerged vegetation. Examples of plants they readily consume include hydrilla, coontail, pondweed, slender spikerush, elodea, and duckweed.
The feeding habits of grass carp can change based on factors such as their age and the availability of their preferred food sources. Younger grass carp tend to be more effective at consuming vegetation and grow more rapidly than older fish. Their consumption rates are highest when water temperatures range between 78 and 90 degrees Fahrenheit, significantly decreasing when temperatures fall below 55 degrees Fahrenheit.
Effectiveness Against Lily Pads
While grass carp are plant-eaters, lily pads (Nymphaea spp. and Nuphar spp.) are generally not a preferred food source for them. The physical characteristics of lily pads, such as their tough stems and floating leaves, make them less appealing and more difficult for grass carp to consume compared to their softer, submerged dietary staples.
Grass carp will typically only resort to eating lily pads when more desirable submerged vegetation is scarce or completely unavailable in their environment. Relying on grass carp solely for the control of lily pads often proves ineffective, or it necessitates very high stocking rates that can lead to other management challenges.
Considerations for Pond Management
When considering grass carp for aquatic plant management, consulting with local aquatic plant specialists or extension services is a beneficial first step. Proper stocking rates are important to achieve desired control without causing unintended ecological imbalances. Stocking rates can vary, typically ranging from 2 to 15 fish per acre for general control, though higher densities of 10 to 40 fish per acre may be recommended depending on the specific plant species and its biomass. Overstocking can lead to the complete eradication of beneficial native vegetation, potentially affecting other fish populations and contributing to algae blooms.
Many regulatory agencies require the use of sterile, triploid grass carp to prevent unintended reproduction in natural waterways. Obtaining a permit is often a prerequisite for stocking these fish. Grass carp offer a long-term control solution, with effectiveness typically lasting 5 to 7 years, and potentially up to 10 to 15 years in some cases if plant eradication is complete. Installing barriers at pond outflows is also important to prevent the fish from escaping into other water systems. For specific issues like lily pad control, other management methods may be more appropriate or can be combined with grass carp for better results.